Mysterious lifeforms known as "Fukashigi" are attacking people. Sora Otosumi, the Mizuki family’s personal Meister, fights the Fukashigi every day to protect high school student Yuna Mizuki. Meanwhile, the Zyunishi Doumei—an organization led by Kamuri Minetaka—resumes its activities after awakening from a 100-year slumber to protect people from the Fukashigi. Amidst this, a new incident involving the "Fukashigi" occurs, and Tatsuomi Asao gets caught up in it. Sora, who rushes to the scene, transforms into Kamen Rider My-Th and confronts the "Fukashigi"!